1. Field of the Invention
The present invention relates to an electrically-conductive liquid for directly printing an electrical circuit component onto a substrate, and a method for making such a liquid. In particular, the present invention relates to an electrically-conductive liquid which, when printed onto a substrate, is capable of carrying out an electrical circuit function, as printed, without the need for post-printing steps such as metal etching, catalytic ink activation, and/or electroless deposition.
2. Related Art
A wide variety of products contain electrical circuitry for carrying signals and/or power to perform communication, display, heating, computation, etc. Such electrical circuitry may be wired by hand, but is typically embodied in a printed circuit board installed in the product. Conventional printed circuit boards comprise a rigid, non-conductive substrate upon which conductive pathways (e.g., metal) have been formed.
Heretofore, a wide variety of processes have been used for forming the conductive pathways on the non-conductive substrate. For example, a metal film such as copper can be applied to a rigid, non-conductive substrate such as fiberglass and epoxy. In a commonly-employed process, a sheet of the conductive metal is laminated to the non-conductive substrate and a photoresist is then coated on the metal sheet. The resulting printed circuit board is subsequently exposed to a light pattern using a light mask to reproduce the metal pathway pattern desired. This exposure is followed by photoresist development and then metal etching in the area unprotected by the photoresist, thereby producing the desired circuit pattern. Alternatively, an etch resist can be directly printed such as by silk screen, gravure, or the like, on the metal laminate sheet followed by curing and then metal etching. Of course, this multi-step process is slow, time-consuming, labor-intensive, and quite expensive.
Another presently available process utilizes metals or metal salts dispersed as particles in a solution, usually in a polymeric binder, and the particles function as seed sites for subsequent plating with a metal. The polymeric composition containing the metal or metal salt is applied to a substrate in a desired pattern. The composition is then heat-cured in order to drive off solvent and to cross-link the polymer. The substrate is then submerged in a metal bath or solution where metal pathways grow between the seed sites. This multi-step process is also slow and expensive. Typical examples of such processes are disclosed, for example, in U.S. Pat. Nos. 3,900,320; 3,775,176; and 3,600,330.
It has also been proposed to form electrically conductive metal pathways by a process which includes coating a substrate with a composition containing a reducible metal complex. In one such process, a substrate is coated with a sorbitol copper formate solution containing a photo-activated reducing agent. Upon exposure to ultraviolet radiation, unmasked areas are reduced to copper metal and are suitable for plating nucleation sites. Non-exposed areas are washed clean and all copper formate is removed before plating can be carried out. Again, much time and expense are involved in such processes. Examples of this technology may be found in U.S. Pat. Nos. 4,268,536; 4,181,750; 4,133,908; 4,192,764; 4,167,601; and 3,925,578.
It is also known to produce a printed circuit board by silk-screen processes in which a silk screen is placed on top of a rigid substrate and ink is pushed through open areas of the silk screen onto the substrate. This, however, is not a direct printing process since the silk screen stencil must first be placed over the substrate, a high viscosity ink is then pushed through the screen onto the substrate, and then the silk screen stencil must be removed. There are several inherent problems in this process. Initially, the inks must be fixed so as not to flow through the screen except where pushed, yet they must be applied with sufficient quantity and thickness of ink to flow together after being applied to make a uniformly conductive surface. Furthermore, the speed of production is very slow with only a small quantity of printed circuit boards being produced within a given period of time. In addition, the precision of the circuits is quite low since pulling the stencil away from the substrate causes dispersion at the edges. Silk screen processing cannot produce thin or narrow lines because of the high viscosity ink. Also, silk screen inks are quite expensive and difficult to process. Silk screen processes cannot be used to produce multi-layer printed circuit boards, and silk screen processes can only be used with rigid substrates since a firm backing is required to push the ink through the silk screen stencil and to remove the stencil. Even if these difficulties can be overcome, silk-screening is difficult or impossible to automate fully for high speed printing.
In another known process, catalytic inks are applied to a substrate and are subsequently activated for plating by electroless surface treatment with noble and/or non-noble metals or salts. Typical examples of these processes are shown in U.S. Pat. Nos. 4,089,993; 4,073,981; 4,100,038; and 4,006,047. The coated substrates are heat-cured and require elevated temperatures and long cure times. Again, such multi-step processes are quite expensive and time consuming.
A technique for producing a flexible anti-theft tag employing a resonant circuit is disclosed in U.S. Pat. No. 3,913,219. According to this technique, an electrically insulated substrate has sheets of conductive foil bonded to each surface thereof. The laminated substrate is next printed on both surfaces with a black carbon ink in the particular patterns required for resonant circuit anti-theft tags. The substrate is then etched to chemically remove all of the unprinted aluminum foil on both sides of the web. The resultant conductive patterns on both web surfaces may be interconnected by welding or by coupling together the conductive surfaces. The time and costs associated with such a technique are readily apparent.
In summary, known processes for producing electrical circuitry such as silk-screen, catalytic ink, chemical etching, electroless bath, etc. are expensive and time consuming. What is needed is a method and apparatus for inexpensive, high-speed production of electrical component-bearing substrates, and especially printed circuit boards.